For touch user interface the sensing of touch can be divided into two stages: Firstly, to detect if the interface is touched and secondly recognize the touch event being press, swipe or any type of action made by a user. Such a touch user interface can be realized by using piezoelectric sensor elements each element corresponding to a discrete touch point of the user interface. In the simplest case each piezoelectric sensor element may correspond to a touch switch. In many user interfaces there is a need to plurality of discrete touch switches or touch points, as an example up to 100 or more for QWERTY keyboards.
Now, if the electric signals of many piezoelectric sensor elements are monitored for touch detection in touch user interface, then as one configuration either one monitoring apparatus per piezoelectric sensor element is required, or as alternative configuration many piezoelectric sensor elements are connected to one monitoring apparatus through a multiplexer. In the first configuration the current consumption of such apparatus increases with increasing the amount of piezoelectric sensor elements in the apparatus, since each monitoring sample in the apparatus having multiple piezoelectric sensor elements requires current to operate. In the second configuration each piezoelectric sensor element must continuously be polled by selecting it with the multiplexer to collect monitoring sample. The continuous polling requires current to operate, also in the inactive stage of the apparatus, when the piezoelectric sensor elements are not producing any signal. This is needed for the apparatus to be able to detect when any one of piezoelectric sensor element starts to produce the signal and consequently apparatus can be changed to active stage for touch detection.
In both configurations, of apparatus being equal amount of monitoring apparatus and piezoelectric sensor elements and configuration of multiplexing piezoelectric sensor element signals to single monitoring apparatus, there are constant and significant energy consumption of the apparatus in inactive stage.
Also in both configurations the energy consumption of the apparatus significantly increases with the amount of piezoelectric sensor elements.
In the first configuration of the apparatus mentioned above, the number of wires and the number of connections required to connect electrically each of the piezoelectric sensor elements, increase linearly with the number of piezoelectric sensor elements required to be monitored. As a consequence, apparatus has complex and bulky mechanical and electromechanical structure, which turns into increased manufacturing cost, lower reliability and limited density of the piezoelectric sensor elements in the user interface since large number of circuit elements and wires are required to electrically connect each of the piezoelectric sensor elements.
In the second configuration of the apparatus mentioned above, the multiplexing, or alternatively time interleaved detection of signal originated from each piezoelectric sensor element, there are less wires and components and space required to electrical connections. On the other hand polling of all the piezoelectric sensor elements in the apparatus may turn out to be too time consuming for reliable touch signal detection due to the limitations of polling speed of all piezoelectric sensor elements, especially with the long settling times required for signals in high impedance measurement. Acceleration of the polling speed can be used to improve the signal detection reliability, but it results in increased power consumption of the apparatus and the performance requirements for the touch detection circuitry, for example sampling speed and accuracy required from the analog-to-digital converter in touch detection circuitry.
It is also possible to use commonly known matrix detection method, where there are active signal fed to the matrix of piezoelectric sensor elements consisting of columns and rows, and as the response to the fed signal there is detection of the change in the signal transfer characteristics as a function of touch at one or more of the switches or touch points. In this alternative of the detection apparatus feeding the active signal would require even higher electrical energy compared to the polling in multiplexed signals and the feeding the signal should be in continuous manner regardless is the user interface touched or not. As a result the electrical power consumption of the apparatus is too high for typical requirement of the touch keypad user interfaces.
Piezoelectric sensing based on charge accumulated by deforming the piezoelectric sensor element is proven to be the most energy efficient method for touch sensing (for example patent reference U.S. Pat. No. 8,810,105). However, there is no reliable and fast enough detection method of touch detection in keypads with plurality of touch switches.